Background and Introduction

Microscale colloids with anisotropic morphologies have proven useful to the development of novel materials and fluids for optical, biomedical, and self-assembly applications. At Harvard University, researchers in Professor Weitz's research group are bestowing anisotropic colloids with Janus-like chemical functionalities to create "colloidal surfactants."

A brief summary of the article cited above is presented along with some demonstrations of the surfactant-like properties of the anisotropic colloids developed by in Professor Weitz's lab.

Summary

Fig. 1, reproduced from Figure 1 of [1]

Fig. 2, reproduced from Figure 5 of [1]

The authors synthesized anisotropic microparticles with dumbbell morphologies that were composed of (or combinations of) polystyrene (PS), polymethyl methacrylate (PMMA), and polybutyl methacrylate (PBMA). The cross-linked PS regions of the anisotropic colloids could be surface functionalized after synthesis with glycidyl methacrylate (GMA), 9-vinylanthracene, and vinylsilane. The synthesis scheme for the colloids' synthesis is given in Figure 1.

Briefly, following a previously reported protocol, spherical microparticles of cross-linked PS (noted as CPS in the paper's figures) were synthesized and swollen for 10 hours to a diameter of approximately 2.7 microns in a solution containing MMA/BMA/styrene monomer (20 vol %), divinylbenzene (DVB, 1 vol %), and the azo polymerization initiator V-65B (2,2'-azodi(2,4'-dimethylvaleronitrile)) at room temperature. The monomer-loaded PS beads were then polymerized at 70 degrees Celsius for 8 hours. During this polymerization step, elastic stress in the swollen PS microparticle drives a phase separation between the PS of the initial spherical particle and the monomer within: a new bulb of monomer nucleates on the surface of the PS particle, initiating the formation of a newly formed polymer bulb. When the initiator is present, nearly all available monomers in solution polymerize at this new bulb. The polymer within the PS particles breach the PS particles' surfaces at one favorable location (i.e. a route of lease resistance) because this phase separation is stress-driven (and in the case of MMA/BMA, the growing polymer is not miscible with PS), resulting in anisotropic colloidal particles with only two bulbs. At the end of the 8-hour polymerization period, the anisotropic colloidal particles are rigid, with one bulb containing mostly PS and the other containing mostly polymer of the previously added monomer. For PS+PMMA/PBMA particles, the PS bulb is hydrophobic, and the PMMA/PBMA bulb is hydrophilic, making the particle amphiphillic. The authors could tune the size of the polymer bulb opposite the cross-linked PS bulb by modifying the ratio of PS particle concentration to monomer concentration in solution for the room temperature swelling step.

When styrene monomer was used, the authors created a hydrophobic PS dumbbell. By initially functionalizing the cross-linked PS particle with GMA, this all-PS dumbbell had one surface-functionalized, cross-linked PS bulb and one "bare" PS bulb. The authors were able to make the GMA-functionalized bulb hydrophilic by binding hydrophilic poly(ethylene iminie) polymer chains (MW = 8 kDa) to the free GMA groups. Thus, like the PS+PMMA/PBMA colloids, the resulting anisotropic PS particle was effectively amphiphillic.

Discussion and Relevance to Soft Matter

Surfactants are amphiphillic molecules with a polar head group and non-polar tail group. This amphiphillicity allows surfactants to self-assemble into interesting structures in solution, including spherical micelles, bilayers, and branched networks.

The anisotropic colloidal particles synthesized by the authors are amphiphillic, like surfactants, and the authors demonstrated that these particles can self-organize into assemblies similar to those formed by surfactants. In the particular paper discussed in this entry, the amphiphillic PS-PS+PEI colloids assembled at an oil-water interface. Figure 2 shows water encapsulated within a spherical interface formed by PS-PS+PEI particles in a water and 1-octanol mixture.

In another article by the same first author, similar anisotropic colloidal particles formed micelle-like structures (see Figure 3).

As anisotropic particles like those presented have surfactant-like properties, the authors propose that "colloid surfactants" could offer routes for improving the stability of emulsions, foams, and other complex fluids, all materials of interest to the field of soft matter.